1. Field of the Invention
The present invention relates to pumping apparatus that minimizes variations in the flow rate of the material that it pumps, in general, and to such apparatus for so controlling the flow rate of a coating fluid, in particular.
2. Description of the Prior Art
In the operation of present-day coating apparatus where coatings of critical thickness are desired to be deposited upon a substrate such as a moving web for use in photographic film, it is necessary to precisely control the flow rate of liquid coating material from a liquid storage reservoir to a coating applicator such as an extrusion coater, which is one device that applies coating material to such a web. An extremely important factor in the control of coating thickness and variations in coating thickness is the control of the flow rate of the material of which said coating is formed, as said coating is being deposited on a substrate.
One piece of apparatus is present use that is able to control the flow rate of coating materials to within acceptable limits utilizes a substantially overpressurized coating material reservoir that has means for regulating the pressure of the coating material flowing from said reservoir. While this type of apparatus will produce a relatively smooth flow rate of material, the flow is not steady due to the limited capacity of such reservoirs. If the flow rate of the material being deposited on a substrate could be maintained for extended periods of time, the production rate of coated finished products such as photographic film, would be substantially increased.
Apparatus for pumping semifluid materials such as a plaster mixture or the like from a reservoir to a use point are presently available. In one system, which is described in U.S. Pat. No. 3,025,803 To SWARTHOUT, a pair of cylinder/piston pumps, having overlapping pumping cycles, alternately deliver material to said use point. A problem with this type of pumping apparatus is the unacceptably high variations in the flow rate of the material being pumped during that portion of the pumping cycle where there is a changeover from one cylinder/piston pump to another, partially because of the suddenness of said changeover, and therefore such apparatus would be unable to provide the constant flow rate that is required in order to obtain the desired coating thickness mentioned above.